The present invention is concerned with measurement of physical phenomena, properties or conditions such as values of resistance, capacitance or inductance, for example, and is more particularly concerned with apparatus and methods for direct digital measurement of minute changes in electrical properties that correspond to changes in related physical phenomena such as mechanical strain, displacement or temperature, for example.
Strain gages and resistive temperature sensors are inexpensive and have been widely used for decades for mechanical design, test and measurement. Various configurations of strain gages are widely employed to measure pressure, weight, torque and mechanical displacement in addition to the direct measurement of strain in engineering design and test. Far more widespread use of such sensors for monitoring, detection and warning of conditions that could, for example, indicate an impending failure has been impeded by the cost, and, to some extent, the size and complexity, of electronics required to convert resistance values to useful measurements. Similar considerations have also limited more extensive use of capacitance and inductance sensors.
Measurements of physical phenomena that produce resistance or reactance changes typically employ a four-element Wheatstone bridge comprising a combination of one or more sensor elements with fixed-value passive components. Such a network is termed a quarter-, half- or full-bridge configuration depending upon whether one, two or all four of the elements are sensors. The Wheatstone bridge configuration is ideal for maximizing measurement sensitivity. This is accomplished by selecting component values to provide zero output for the unstressed or no-load condition, allowing the bridge output signal to be greatly amplified.
In the prior art, a precisely-controlled, fixed DC voltage is applied to the Wheatstone bridge or other network containing the sensing elements. The output is then amplified, with additional circuitry employed to cancel DC offset errors introduced by the amplifier circuits. The amplified DC analog signal is then applied to an analog-to digital converter to obtain a digital output. This approach involves considerable cost and complexity and is susceptible to errors due to lead length, noise pickup and other error sources such as ambient temperature variations.
It is often required to measure outputs from multiple independent sensors, either simultaneously or in a repeating sequence. Prior art teaches the use of analog multiplexers, sample-and-hold circuits, together with associated control circuits such as scan, trigger, and address decode to implement such multi-channel systems. This additional cost and complexity further limits more widespread use of low-cost passive sensors.
It is accordingly an object of the present invention to provide an apparatus and method for precise, high-resolution direct digital measurement of phenomena which produce changes in electrical properties such as resistance, capacitance and inductance.
Another object of the invention to provide an apparatus and method of the above-described type capable of multiple independent, simultaneous measurements using a single reference oscillator and logic device.
Another object of the invention is to minimize noise sensitivity in an apparatus and method of the above-described type by employing synchronous detection of low-frequency sensor signals to achieve extremely narrow noise bandwidth.
Another object of the invention to provide an apparatus and method of the above-described type which is low in cost.
Another object of the invention is to minimize errors due to temperature and voltage variations in an apparatus and method of the above-described type through the use of excitation and reference signals derived from a common source.
It is a further object of the invention is to eliminate the need for DC offset correction by employing AC excitation.
Briefly stated, the present invention provides high-accuracy, high-resolution apparatus, systems and methods that employ phase shifts in a fixed-frequency signal for measurement of electrical properties such as capacitance, inductance or resistance.
In accordance with a preferred embodiment of the invention, two sinusoidal signals of identical frequency and 90 degrees out of phase are generated from a single reference oscillator. One is designated xe2x80x9cexcitation,xe2x80x9d the other, xe2x80x9creference.xe2x80x9d
The excitation signal is applied to a network such as a Wheatstone bridge comprising both fixed-value components and one or more sensing elements. The values of the bridge components are preferably selected to provide minimum voltage output when the sensing elements are unstressed and an output level (amplitude) proportional to stress applied to the sensing elements when a stress such as a force or temperature change is applied.
The output of the bridge is amplified and then applied as one input to a summing amplifier, the other input being the above-described reference signal. All signal levels are constrained to remain within the linear range of amplifier circuitry employed.
As the amplified output of the Wheatstone bridge ranges from zero to a maximum amplitude equal to that of the reference signal, the phase xcex8 of the summing amplifier output with respect to the reference signal will vary from 0xc2x0 to 45xc2x0.
High-resolution measurement of xcex8 is accomplished by applying the summing amplifier output to a unique phaselocked loop mechanization to produce a loop output signal that multiplies phase changes in the summing amplifier output by orders of magnitude. In the loop, the input from the summing amplifier and the loop feedback signal are applied to a phase detector, the output of which is used to produce a loop output signal at a frequency that is a multiple of the summing amplifier input frequency. The loop output signal is used to delete pulses from the reference oscillator signal, which is at a much higher frequency than the loop output signal. The output of the pulse delete circuit is then applied to a divider, the output of which is equal in frequency to the summing amplifier input signal. The result of this arrangement is that phase changes in the summing amplifier input to the loop are multiplied in the loop output by the same constant used to divide down the output of the pulse delete circuit.
The loop output signal is then compared with a signal at the same frequency derived from the reference oscillator using logic that converts cycle slips between the two signals into up/down counts. The resulting up/down counts can then be directly applied to an indicating device, recorder or to a microprocessor or microcontroller.